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Background  SWEET is a newly identified family of sugar transporters. Although SWEET transporters have been characterized by using Arabidopsis and rice, very little knowledge of sucrose accumulation in the stem region is available, as these model plants accumulate little sucrose in their stems. To elucidate the expression of key  SWEET  genes involved in sucrose accumulation of sorghum, we performed transcriptome profiling by RNA-seq, categorization using phylogenetic trees, analysis of chromosomal synteny, and comparison of amino acid sequences between SIL-05 (a sweet sorghum) and BTx623 (a grain sorghum).    Results  We identified 23  SWEET  genes in the sorghum genome. In the leaf,  SbSWEET8 - 1  was highly expressed and was grouped in the same clade as  AtSWEET11  and  AtSWEET12  that play a role in the efflux of photosynthesized sucrose. The key genes in sucrose synthesis ( SPS3 ) and that in another step of sugar transport ( SbSUT1  and  SbSUT2 ) were also highly expressed, suggesting that sucrose is newly synthesized and actively exported from the leaf. In the stem,  SbSWEET4 - 3  was uniquely highly expressed.  SbSWEET4 - 1 ,  SbSWEET4 - 2 , and  SbSWEET4 - 3  were categorized into the same clade, but their tissue specificities were different, suggesting that SbSWEET4-3 is a sugar transporter with specific roles in the stem. We found a putative  SWEET4 - 3  ortholog in the corresponding region of the maize chromosome, but not the rice chromosome, suggesting that  SbSWEET4 - 3  was copied after the branching of sorghum and maize from rice. In the panicle from the heading through to 36 days afterward,  SbSWEET2 - 1  and  SbSWEET7 - 1  were expressed and grouped in the same clade as rice  OsSWEET11/Xa13  that is essential for seed development.  SbSWEET9 - 3  was highly expressed in the panicle only just after heading and was grouped into the same clade as  AtSWEET8 / RPG1  that is essential for pollen viability. Five of 23  SWEET  genes had SNPs that caused nonsynonymous amino acid substitutions between SIL-05 and BTx623.    Conclusions  We determined the key  SWEET  genes for technological improvement of sorghum in the production of biofuels:  SbSWEET8 - 1  for efflux of sucrose from the leaf;  SbSWEET4 - 3  for unloading sucrose from the phloem in the stem;  SbSWEET2 - 1  and  SbSWEET7 - 1  for seed development;  SbSWEET9 - 3  for pollen nutrition.    Electronic supplementary material  The online version of this article (doi:10.1186/s13068-016-0546-6) contains supplementary material, which is available to authorized users.

The sorghum SWEET gene family: stem sucrose accumulation as revealed through transcriptome profiling